Hindawi Publishing Corporation
Journal of Sensors
Volume 2015, Article ID 983697, 6 pages
http://dx.doi.org/10.1155/2015/983697
Research Article
Towards a Carbon Nanotube Intermodulation Product Sensor
for Nonlinear Energy Harvesting
Mitchell B. Lerner, Brett Goldsmith, John Rockway, and Israel Perez
Functional Nano Devices Lab, System of Systems and Platform Design Division, SPAWAR Systems Center Pacific, 53560 Hull Street,
San Diego, CA 92152, USA
Correspondence should be addressed to Israel Perez; israel.perez2@navy.mil
Received 30 October 2014; Revised 3 May 2015; Accepted 11 May 2015
Academic Editor: Jiri Homola
Copyright © 2015 Mitchell B. Lerner et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
It is critically important in designing RF receiver front ends to handle high power jammers and other strong interferers. Instead
of blocking incoming energy or dissipating it as heat, we investigate the possibility of redirecting that energy for harvesting and
storage. The approach is based on channelizing a high power signal into a previously unknown circuit element which serves as
a passive intermodulation device. This intermodulation component must produce a hysteretic current-voltage curve to be useful
as an energy harvester. Here we demonstrate a method by which carbon nanotube transistors produce the necessary hysteretic
I-V curves. Such devices can be tailored to the desired frequency by introducing functional groups to the nanotubes. These effects
controllably enhance the desired behavior, namely, hysteretic nonlinearity in the transistors’ I-V characteristic. Combining these
components with an RF energy harvester may one day enable the reuse of inbound jamming energy for standard back end radio
components.
1. Introduction
Wideband receiver designs used in command and control
systems may be susceptible to high level, off-tuned interference [1, 2]. Specifically, a high power jammer or other sources
of strong interfering signals in nearby frequencies will overload the amplifier and limiter stages in the front end of such
communications receivers. For example, frequency hopping
receivers in very close proximity to multiple high-powered
transmitters have their communication range reduced significantly [3–5]. This range reduction is primarily due to nonlinear effects in the front end of the receiver, whereby diodes
near the receiver’s antenna port typically used for power limiting or circuit switching begin to behave as mixers [6]. Mixing leads to significant intermodulation products in virtually
all channels [7].
Typically the effects of an interferer are limited by channelizing the signal [8, 9]. One such technology is the Comb
Limiter Combiner (CLiC) filter, which is able to neutralize a
strong interferer by distributing an antenna signal to a bank
of input bandpass filters [10, 11]. The input bandpass filters
have contiguous passbands that comprise the total receiver
bandwidth. Each input bandpass filter is connected to a
limiter having a threshold equal to the limiting threshold
of the receiver, while each limiter is connected to an output
bandpass filter similar to the corresponding input bandpass
filter to remove out-of-band intermodulation products. The
bank of output bandpass filters is connected to an output
signal coupler for coupling to the front end of the receiver. The
advantage of the CLiC is that intermodulation products are
restricted to the passband of a single bandpass filter. However,
it is ultimately limited by heat buildup and power handling of
the individual filter elements [12, 13].
Passive intermodulators, such as rusty bolts on a ship,
can form isolated resonant cavities for the production of
intermodulation products, which then reradiate back into
the original antenna or circuit (Figure 1(c)) [14]. A hysteretic
current-voltage characteristic is required for a resonator to
produce intermodulation products [15, 16]. When a twotone, high power signal interacts within the cavity, the tones
interfere in a manner dependent on both the magnitude and
the frequency of the tones. If the 𝐼-𝑉 were nonhysteretic
2
Journal of Sensors
I
𝜔1
𝜔2
U
(a)
(b)
0
−20
Power (dB)
−40
−60
−80
−100
−120
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Frequency (GHz)
PIM model
Memristor with nearby strong interferer
Input
Output
(c)
Figure 1: (a) Rusty bolt on a ship hull, which can act as a passive intermodulator. (b) Current-voltage characteristic of a memristor, a nonlinear
circuit element, displaying hysteretic, frequency-dependent behavior. Such properties are critical for creating intermodulation products. (c)
A simulated two-tone input produces strong intermodulation products at many different frequencies due to the nonlinear memristor circuit
element.
(i.e., a resistor), the tones would not self-interfere and hence
could not produce intermodulation products.
Carbon nanotube-based devices may help to mitigate the
effects of high power jamming by intentionally shifting some
of that energy into unused frequency bands in the form of
out-of-band intermodulation products [17]. Further, instead
of blocking the out-of-band energy or dissipating it as heat
in filters or amplifiers, an alternative approach is to harvest
the energy for powering low profile devices [18, 19]. Carbon
nanotubes have unique electronic and optoelectronic properties which make them viable candidates for producing outof-band intermodulation products, which can then be routed
into an energy recycler [20]. The first step in producing
such a device is to create a circuit element that displays a
hysteretic, controllable current-voltage trace. The hysteretic
current-voltage curve produced by applying a slowly varying
AC back gate voltage to a carbon nanotube transistor is just
such a component to allow for selective frequency shifting
of high power signals in a nondissipative manner. Chemical
defects made to the nanotube sidewall change the resistance
of the device, thus modifying the hysteretic current-voltage
characteristic in a controllable manner. The carbon nanotube
intermodulation (CNTIM) system could serve as a replacement for the traditional CLiC limiter circuit, where high
power signals are splattered to intermodulation frequencies
with the CNTIM device, and the resulting energy is given a
low resistance path to a power collector through low pass and
high pass filters.
2. Materials and Methods
Experiments were performed using carbon nanotubes grown
by chemical vapor deposition (CVD) according to a published recipe [21]. Briefly, iron nitrate was dissolved in isopropanol and deposited using a spinner onto a 500 nm SiO2 /
Si wafer. As the temperature in the CVD furnace increased to
900∘ C, the iron coalesced into nanoparticles, which served as
Journal of Sensors
3
SiO2
Si
Source
A
Drain
Vsd
Vg
Pristine carbon nanotube transistor
O
O
O
S
S
O
O
S
S
S
+
O
O
O
O
O
n
x
y
SiO2
Si
Source
A
Drain
Vsd
O S O O S O
O−
OH
Vg
PEDOT
After PEDOT defects
(a)
100
ISD (nA)
80
60
40
20
0
−10
−5
0
5
10
VGS (V)
Before PEDOT
After PEDOT
(b)
(c)
Figure 2: (a) Carbon nanotube transistor before and after PEDOT : PSS attachment. The chemical locally gates the nanotube transistor (shown
in red) resulting in a change in the electronic transport properties. (b) Carbon nanotube transistor being measured on a probe station. (c)
Source-drain current as a function of the back gate voltage for a carbon nanotube transistor before and after PEDOT : PSS functionalization.
The ON state current decreases and the transistor turnoff voltage shifts to the left, indicating the presence of a charged chemical moiety.
the catalyst for the decomposition of methane and the formation of carbon nanotubes. Source and drain electrodes
were patterned such that each transistor device consisted of
a sparse network of roughly twenty nanotubes. Responses of
a nanotube ensemble have been shown to be more robust
than single nanotube devices [22, 23]. Chemical defects were
created by spin casting a solution of PEDOT : PSS diluted 1 : 9
with deionized water at 3000 rpm for 1 min.
4
Journal of Sensors
3.0
4
2.5
3
ISD (𝜇A)
ISD (𝜇A)
2.0
1.5
2
1
1.0
0
0.5
0.0
−1
0
1
2
3
4
5
0
1
2
3
4
5
VSD (V)
VSD (V)
Before PEDOT Vg = −8 V
After PEDOT Vg = −8 V
Before PEDOT Vg = −8 V
Before PEDOT Vg −10 V to −6 V
(b)
(a)
2.0
3.0
1.5
Hysteresis width (𝜇A)
2.5
ISD (𝜇A)
2.0
1.5
1.0
1.0
0.5
0.0
0.5
0.0
−0.5
0
1
2
3
4
5
VSD (V)
Before PEDOT Vg = −8 V
Before PEDOT Vg −8.5 V to −7.5 V
Before PEDOT Vg −9 V to −7 V
Before PEDOT Vg −9.5 V to −6.5 V
Before PEDOT Vg −10 V to −6 V
After PEDOT Vg = −8 V
After PEDOT Vg −8.5 V to −7.5 V
After PEDOT Vg −9 V to −7 V
After PEDOT Vg −9.5 V to −6.5 V
After PEDOT Vg −10 V to −6 V
(c)
0
1
2
ΔVg (V)
Before PEDOT: positive VSD
Before PEDOT: negative VSD
3
4
After PEDOT: positive VSD
After PEDOT: negative VSD
(d)
Figure 3: (a) Current-voltage (𝐼-𝑉) measurement of a single device before and after PEDOT : PSS functionalization. (b) 𝐼-𝑉 measurement
of a single device taken with the back gate fixed (sweep frequency = 0 Hz) and with an AC voltage applied (1 Hz). (c) 𝐼-𝑉 curves taken before
and after chemical functionalization on a single device with varying magnitudes of back gate modulation. (d) Summary plot of the hysteresis
width as a function of the applied back gate modulation.
3. Results and Discussion
At this concentration, the density of PEDOT : PSS on the surface was insufficient to be conductive on its own, but enough
molecules remained to coat the nanotube surface via 𝜋-𝜋
stacking (Figure 2(a)). Several carbon nanotube transistors
were measured on a homebuilt probe station (Figure 2(b))
and the DC source-drain current as a function of applied back
Journal of Sensors
gate voltage confirms the attachment of the PEDOT : PSS
compound. A CNT device is depicted in Figure 2(c) with
an ON state current of 92 nA and an ON/OFF current ratio
greater than 4200. After PEDOT : PSS functionalization, the
current is reduced by ∼10–15% and the turnoff voltage of the
transistor typically decreases 1.0–1.5 V. The current reduction
is likely due to the PEDOT : PSS acting as a scattering center
for charge carriers in the nanotube while also locally gating
the device. Indeed, the negative charges of the sulfonate
groups would be protonated at the SiO2 /water interfacial pH
of 5, leaving only the positive charge of the ethylenedioxythiophene group which would produce the gating effects
observed [24, 25].
Measurements of the 𝐼sd -𝑉sd curve for a given carbon
nanotube transistor were taken with the gate voltage (𝑉𝑔 )
fixed at −10 V so that the device was in the conductive state. As
demonstrated in Figure 3(a), there is not any significant hysteresis in the 𝐼-𝑉 curve for either the pristine device or the
PEDOT : PSS coated device. However, there is a clear rotation
in the 𝐼-𝑉 plane due to the new device resistance after attachment of PEDOT : PSS. To control the hysteresis width, we
applied a time varying back gate voltage with known amplitude where the time scale of the gate voltage cycling is comparable to the measurement sweep rate. A back gate swing
of 4 V caused a widening of the hysteresis by ∼1.5 𝜇A
(Figure 3(b)). The amount of hysteresis induced is controllable through the magnitude of the back gate swing as well
as the presence or absence of chemical functionalization
(Figure 3(c)). A concise summary of the relation between
gate swing and hysteresis width for 12 devices is plotted in
Figure 3(d). Materials with 𝐼sd -𝑉sd curves like those generated
in Figure 3(c) are suitable for producing intermodulation
products as discussed. By applying a high power signal to the
input of the carbon nanotube transistor, the resulting sourcedrain current should contain energy at mixed frequencies.
The input power required for this device is the bias voltage
(1 𝜇W). A typical linear amplifier produces as much as 5 mW
of energy at off-resonant frequencies, meaning the potential
gain in energy for this device configuration is a factor of 5000.
4. Conclusions
In sum, we have demonstrated a nonlinear circuit element
made from carbon nanotube transistors which display hysteretic effects. This type of hysteretic circuit element is required to produce intermodulation products from a high power
input signal. By introducing a chemical defect, we can change
the device resistance and access more of the parameter space
for tailoring the sensor to a particular frequency band if
necessary. These devices may one day be incorporated into
a system like the CLiC channelizer for energy harvesting in
unused frequency bands, thus extending the lifetime of low
power devices in the field such as security alarms, microphones, power meters, and low power radar.
Conflict of Interests
The authors declare that there is no conflict of interests
regarding the publication of this paper.
5
Acknowledgment
Support from SSC Pacific NISE Internal Funding is gratefully
acknowledged, as is use of its facilities.
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